Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cor tubular, funnelform, or campanulate, with a definite distinction between tube and limb, the tube short or elongate, the limb shallowly lobed; stamens inserted at the summit of the cor-tube (in ours), included or barely exsert; nutlets smooth or often wrinkled, attached laterally to the gynobase; leafy perennials with blue (pink or white) fls in modified, bractless, mostly small cymes ending the stem and branches. 35–40, N. Amer. and extratropical Eurasia.